Caption: Ectomorphic body type shown here. Constitutional psychology is a theory, developed in the 1940s by American psychologist William Herbert Sheldon, associating body types with human temperament types. Sheldon's 'somatotypes' and their supposed associated physical traits can be summarized as follows: Ectomorphic: characterized by long and thin muscles/limbs and low fat storage; receding chin, usually referred to as slim. Mesomorphic: characterized by medium bones, solid torso, low fat levels, wide shoulders with a narrow waist; usually referred to as muscular. Endomorphic: characterized by increased fat storage, a wide waist and a large bone structure, usually referred to as fat.